Most electric vehicles and concepts displayed at the 2010 Paris Motor Show were based around low-production-cost models, which means a single drive motor. For its C-X75 concept, Jaguar took a far more experimental approach. The car is based around a two-seat high-performance coupe built using Jaguar’s aluminum spaceframe construction. The drive system is based around a range-extender plug-in hybrid-electric architecture. The electric drive system uses wheel motors in each wheel to give four-wheel drive with torque-vectoring capability.
“Having it as a pure EV, we don’t think is viable because you’d have to stack the car from front to back with batteries,” Tony Harper, head of research and advanced system engineering at Jaguar told AEI. “Even then, you wouldn’t get much more than a 200 mi range and certainly not if you tried to drive it as a supercar. So we looked to size the battery at about a 70-mi range, which covers what most people do with a car most of the time. In capacity terms, it’s about a 19-kW·h battery.
“But we want this to be a supercar both in terms of its performance and its range," Harper continued. "To make that happen, you essentially need a power extender and a range extender that is very power dense. You could use a reciprocating engine or a rotary engine in this solution, but what you don’t want to do is compromise what most people are doing most of the time, which is driving this car as an EV. With either of those solutions you’d be carrying a lot of weight and package around when you don’t use it. In fact you’d be penalized for lugging that capability around. So that drove us to looking for lightweight, power-dense range extension and power extension solutions and that takes you fairly quickly to gas turbines.”
So the C-X75 is equipped with two micro gas turbines producing 94 hp (70 kW) at 80,000 rpm and supplied by Bladon Jets, a company in which Jaguar Land Rover’s owner Tata took a minority shareholding during the Paris Show. Each turbine, combined with the switch reluctance generator, weighs 35 kg (77 lb) each.
“Normally the problem with small gas turbines [that are] so small is that you’re struggling with efficiency, which is a function of compression ratio," said Harper. "Trying to get a high compression ratio in small gas turbines is an issue. One of the breakthroughs we’ve had is working with Bladon Jets, who have been able to produce a set of axial compression stages in a micro gas turbine that give you a pretty high compression ratio.
“They get you not quite to the efficiencies of an internal combustion engine, but again if you take into account that this is an EV most of the time, that’s possibly a compromise that we’re willing to take," Harper continued. "So that’s what really drove us to that, plus that fact that we were obviously trying to be a little more stretchy with the technology and stretchy with the design. We could have shown a more conventional range extension type technology but we thought, “No, we have a research program looking at this technology, let’s put it in this concept and signal that as part of our vision for the future.”
Turbines have potential advantages for a range-extender application. They have a small number of moving parts compared with a reciprocating engine and use air bearings, which remove the necessity for lubricating oil. They do not need liquid cooling systems either and can run on a range of fuels including diesel, compressed natural gas, liquefied petroleum gas, and biofuels. Since turbines tend to work efficiently across a small rev range, they are well adapted to the steady speed running in a range-extender system where they will be either recharging the batteries or supplying electricity directly to the drive motors without the transient changes associated with direct drive.
The turbines are coupled to two switch reluctance generators supplied by SR Drives and these either operate sequentially or in tandem depending on power demands. Each of the hub motors weighs 50 kg (110 lb), but can produce 145 kW and 1600 N·m (1180 lb·ft) of torque each. The drive motors also provide regenerative charging to the batteries.
The drive control system can vector the torque to each wheel depending on any given driving situation. Jaguar quotes a total power output of 780 hp (580 kW), with carbon dioxide emissions of 28 g/km from the gas turbine drive system.
With so much drive torque available, Jaguar says it had to limit the torque available from rest to improve driveability. The car is said to be able to accelerate from rest to 100 km/h (62 mph) in 3.4 s, from rest to 160 km/h (100 mph) in 5.5 s, and from 80-145 km/h (50-90 mph) in 2.3 s.
The micro-turbines are mid-mounted behind the passenger compartment in a sealed air box, with air fed to them from channels in the body. The turbines are visible through the back window of the C-X75. At 80,000 rpm, each turbine requires 35,000 L (1240 ft³) of air a minute, which means that the intake and exhaust channels needed careful design. Jaguar has designed a directional control for the exhaust gases, which in combination with an underbody venturi and moveable aerofoil can generate maximum downforce without using spoilers.
We asked about drive system cost-effectiveness, Tony Harper responded. “We’re in an experimental stage with experimental costs. However, if you carry out what we call a zero-based estimate of the parts in the machine, it’s actually pretty cost effective. In fact, if you were to take the bill-of-materials of a state-of-the-art petrol engine coupled with a transmission and look at the inherent cost of that versus the inherent cost of the gas turbines, the cost of the gas turbines is much, much less. Obviously it’s going to take scale to get to those prices, but fundamentally, we don’t see why there is any more than a couple of hundred pounds Sterling in one of those units.”